1. Field of the Invention
The present invention relates in general to apparatus for diagnosing exhaust gas purifying devices and more particularly to an apparatus capable of diagnosing a deterioration of a catalyst even when an air-fuel ratio detector on the downstream side of the catalyst is degraded.
2. Description of Related Art
Many of known apparatus for diagnosing exhaust gas purifying devices detect a deteriorated condition of the catalyst based on the relation between information detected by an air-fuel ratio detector disposed on the upstream side of the catalyst and that detected by an air-fuel ratio detector disposed on the downstream side of the catalyst. One known apparatus, for example, uses information detected by an air-fuel ratio detector disposed upstream of the catalyst and that detected by an air-fuel ratio detector disposed downstream of the catalyst to calculate a ratio between a cross-correlation function and an auto-correlation function and detects a deteriorated condition of the catalyst based on that ratio (see, for example, JP-A-5-171924). Some other known apparatus uses as a catalyst deterioration index a period ratio between the information detected by the air-fuel ratio detector disposed upstream of the catalyst and that detected by the air-fuel ratio detector disposed downstream of the catalyst or a ratio of the numbers of fluctuations occurring during a given period of time.